High-pressure plunger pumps used in oil and gas fields, particularly those intended for fracking, are usually designed in two sections. The (proximal) power section (herein “power end”) and the (distal) fluid section (herein “fluid end”) are often truck-mounted for easy relocation from well-to-well. The fluid end comprises a housing incorporating one or more functional units, each functional unit typically comprising a suction valve, a discharge valve, and a plunger bore in which a reciprocating plunger alternately produces suction strokes and pressure strokes. Each functional unit also incorporates a packing box containing plunger packing assemblies analogous to packing assemblies shown in U.S. Pat. No. 4,572,519, incorporated by reference. Adjacent packing rings, as shown in the '519 patent, have corresponding chevron-shaped ends which reduce leakage as the rings are longitudinally compressed between a proximal bronze adapter ring and a distal adapter ring.
Packing rings thus longitudinally compressed can seal tightly around a plunger, ideally preventing leaks even at peak pumped-fluid pressures of about 15,000 psi. Because of the high peak operating pressures common in modern practice, packing rings must be made relatively stiff. They may comprise, for example, layered fabric-reinforced rubber. Each chevron-shaped ring end then comprises a concave or convex circumferential feature which is an extension of the ring's layered construction. Corresponding chevron shapes on adjacent packing ring ends allow for modest radial expansion under longitudinal compression to achieve a tight seal around a plunger. Longitudinal compression force, in turn, is provided by a gland nut and by the cyclically increased pressure of the pumped fluid.
Turning the gland nut adjusts longitudinal preload (i.e., baseline longitudinal compressive force) that is applied to the packing ring(s). Such preload is intended to prevent leakage and/or excessive cyclic ring movement under peak pumped-fluid pressures, with their associated pump vibrations. The emergence of pump vibration as a potential service-life issue in high-pressure plunger pumps has focused attention on how vibration affects, among other things, plunger seals' tendency to leak. See, e.g., U.S. Pat. No. 5,639,098, incorporated by reference.
To minimize seal leakage, substantial longitudinal preload is applied to packing rings which have been manufactured to fit very tightly around a plunger even before preload is applied. Thus, even if longitudinal preload is reduced to zero by backing out the gland nut completely, packing rings typically remain tightly sealed around the plunger due to their stiffness. This means the plunger must virtually always be withdrawn from the packing box to allow removal of the packing ring(s), as may be required during pump maintenance. Plunger withdrawal, however, is often complex because of interference between the plunger and various power end components.
Complex pump maintenance is required more frequently in modern practice because the packing rings' high-pressure sealing function requires that the rings substantially retain their original (chevron) end shape. But such ring-shape retention becomes progressively more difficult as frictional heat developed during pump operation softens the packing rings and predisposes them to extrusion under pressure through a gap (the extrusion gap) between the proximal adapter ring and the plunger.
A conflict thus arises when packing ring longitudinal compression is increased to improve the plunger seal, because that increased compression also results in additional frictional heating which degrades the rings' functional integrity. Since dissipating heat within the confines of the packing box is difficult, high pressure plunger pumps are often limited to relatively short periods of operation (e.g., no more than a few hours). Extrusion of the packing ring material through the extrusion gap is thereby limited. But if pump run times are extended, packing rings tend to overheat and fail prematurely.
The above conflict has been partially resolved by replacing the proximal bronze adapter ring with a proximal adapter ring comprising PEEK polymer in certain seals. PEEK (polyetheretherketone) is a high performance thermoplastic, a portion of which moves radially inward under longitudinal compression, tending to narrow the extrusion gap. Such radial inward (gap-narrowing) movement is termed “elongation” in industry advertising and is said to be more prominent in a PEEK adapter ring than in an analogous bronze adapter ring. See, e.g., U.S. Pat. No. 7,847,057 B2, incorporated by reference. While the use of commercially-available PEEK adapter rings appears to increase plunger packing service life, issues related to frictional wear, heat generation and pump vibration remain.